Mission

The U.S. Geological Survey has the principal responsibility within the Federal Government to provide the hydrologic information and understanding needed by others to achieve the best use and management of the Nation's water resources. To accomplish this mission, the Water Resources Division, in cooperation with State, local, and other Federal agencies,

Systematically collects and analyzes data to evaluate the quantity, quality, and use of the Nation's water resources and provides results of these investigations to the public.

• Conducts water resources appraisals describing the occurrence, availability, and physical, chemical, and biological characteristics of surface and ground water. Conducts basic and problem-oriented hydrologic and related research that aids in alleviating water resources problems and provides an understanding of hydrologic systems sufficient to predict their response to natural or human-caused stress.

Coordinates the activities of Federal agencies

in the acquisition of water resources data for streams, lakes, reservoirs, estuaries, and ground water.

• Provides scientific and technical assistance in hydrologic fields to other Federal, State, and local agencies, to licensees of the Federal Energy Regulatory Commission, and to international agencies on behalf of the Department of State.

Administers the State Water Resources Research Institutes Program and the National Water Resources Research Grants Program.

Streamflow affects transport, residence time, and fate of pesticides in the delta. River inflows to the delta and total freshwater discharge from the delta differ by an order of magnitude between extreme years of drought or floods and even seasonally during some years. The operation of Federal and State water projects that transfer water from northern California sources to the San Joaquin Valley and southern California also affects the flow system in the delta. Typically, most San Joaquin River flow during the summer goes to pumping plants; therefore, the Sacramento River is virtually the sole freshwater source to the estuary. In winter, river inflows are significantly higher and the export pumps have less effect on flow patterns in the delta.

Pesticide Sources and Transport

Three major crops, almonds, alfalfa, and rice, were selected for source and transport studies because the pesticides that are used on these crops include compounds having different biogeochemical properties (fig. 2). Pesticides are applied to these three crops at different times during the year, as is clearly illustrated in the graph of monthly application rates of a representative pesticide for each crop in 1988 (fig. 2B). Almond orchards, located primarily along the San Joaquin River, are sprayed with different pesticides, including diazinon, in January and February and again from May through August.

Stockton

RIVER

Mossdale

Vernalis

Pesticides, including carbofuran, are applied to alfalfa fields near the San Joaquin and within the delta in March and to rice fields primarily along the Sacramento River from April through June. After a lag time following application, the pesticides are transported in runoff to the river where they move downstream in river water as a pulse (high concentrations that are detected for short periods of time). Pesticide transport is primarily controlled by the hydrology of the river system, and seasonal distributions of the different pesticides are further influenced by the fluctuations in discharge to the delta.

Biogeochemical properties of pesticides are also important factors that influence their distribution in the environment. For example, if a particular pesticide is dissolved in the

Figure 1. Sampling sites in the Sacramento-San Joaquin Delta, Calif. The major route of water flow in the San Joaquin River from Vernalis to the delta, as indicated by dye studies in February 1991, is past Mossdale, along Grant Line Canal and Old River, and then directly to the Federal and State export pumps.

water, it will move with the water; if the pesticide is a particle-bound compound, it will adsorb onto the surface of a particle and be transported in the water with that particle. How long the pesticide will remain in the environment is determined by the rate at which it degrades, either microbially or chemically, into another compound. The resulting degradation compound can be either more or less toxic than the parent compound, and the rate of its degradation depends on whether the pesticide is dissolved or adsorbed onto a particle.

Almond Pesticides.-The primary pesticides that are applied to almond orchards are dissolved compounds (ethyl parathion and diazinon) and the particle-bound compound chlorpyrifos. Almond pesticide loading to the San Joaquin River occurs primarily in February and March. A pulse of diazinon at a maximum concentration of 80 nanograms per liter was detected from February 3 through 12 at a site at the mouth of the San Joaquin River near Vernalis (fig. 1, lower right). The river was being monitored to determine the timing of the almond pesticide pulse in river water from Vernalis to the delta. Only traces of

A

ethyl parathion were detected in the same samples even though six times as much ethyl parathion was applied to the almond orchards as diazinon because ethyl parathion degrades rapidly in the orchards. Chlorpyrifos was not detected in the water samples.

The concentrations of almond pesticides in water samples from the delta, at the end of the pulse detected at Vernalis, were very low. Only diazinon was detected in the water samples at concentrations just higher than the detection limit of 10 nanograms per liter.

Alfalfa Pesticides.-The primary pesticides applied to alfalfa fields are dissolved compounds (carbofuran, diazinon, malathion, and diuron) and particle-bound compounds (chlorpyrifos, endosulfan, and trifluralin). The alfalfa pesticides reach the delta in March and April from different locations along the San Joaquin River as well as from local deltaic sources. A pulse of diazinon at a maximum concentration of 84 nanograms per liter was detected in the San Joaquin River from March 14 through 27. During this time, concentrations of carbofuran also increased to a maximum of 100 nanograms per liter and traces of chlorpyrifos were detected.

Figure 2. Pesticide application on almonds, alfalfa, and rice in the Sacramento-San Joaquin Delta, Calif. A, geographic extent of pesticide application. B, monthly (1988) application rates and average monthly (1956-85) stream discharge past Chipps Island (see fig. 1).

Other fixed sampling sites included Freeport on the Sacramento River, two sites along Old and Middle Rivers within the delta, and at Chipps Island, the boundary between the delta and San Francisco Bay (fig. 1). Concentrations of diazinon were at the detection limit (10 nanograms per liter) during the sampling period (end of March through mid-May) at these riverine and delta sites and were slightly higher at Chipps Island.

In contrast, concentrations of carbofuran reached a maximum value at all sites on April 5 and decreased thereafter. Carbofuran concentrations increased seaward; maximum levels ranged from 70 nanograms per liter at Freeport on the Sacramento River to 250 nanograms per liter at Chipps Island. For alfalfa and other important delta crops, the information gained from the sampling period shows that pesticide loading to the bay should be measured at the delta-bay boundary at Chipps Island so both riverine and delta sources will be included.

Rice Pesticides.-Rice pesticides enter the delta primarily from the Sacramento River, which is tidal downstream from Freeport. A concentrated load of dissolved pesticides (carbofuran, methyl parathion, molinate, and thiobencarb) from rice fields enters the Sacramento River in May and June from agricultural drains, such as the Colusa Basin Drain (fig. 1). The presence of these compounds. during spawning season and the results of toxicity tests by personnel of State and Federal agencies suggest that the larval stages of striped bass are adversely affected by the pesticides in the river.

The concentrations of the rice pesticides and their degradation products were measured during a 4-day period, beginning June 3, as they were transported downriver from Sacramento to Rio Vista (fig. 1). Three of the four pesticides were detected at all sampling sites: molinate, in concentrations from 7,000 to 12,000 nanograms per liter; thiobencarb, from 40 to 60 nanograms per liter; and carbofuran, from 125 to 350 nanograms per liter. The concentrations of these three pesticides changed little over time as they were transported downstream. Methyl parathion was not detected in any of the river water samples. Samples taken at Chipps Island (fig. 1) also contained measurable concentrations of the three rice pesticides that were detected in the delta water, which suggests that these pesticides are being transported into San Francisco Bay.

Summary

There is much uncertainty about the long-term biological effects of pesticides in the environment; therefore, the standards for concentrations of pesticides in surface waters

are not well established. Whereas some of the dissolved-pesticide concentrations measured during this study are not of concern, concentrations of diazinon in samples taken at Vernalis in 1991, after this pesticide was used on almond orchards and alfalfa fields, were considerably above the U.S. Environmental Protection Agency recommended maximum criterion for aquatic life of 9 nanograms per liter. The factors that control the sources, transport, and transformation of pesticides in surface waters also are poorly understood. In the Sacramento and San Joaquin Rivers, a striking feature is the pulse effect of pesticide loads; that is, high concentrations are detected for only a few weeks at a time.

The application of pesticides on the fields in the Central Valley of California is regulated and recorded, but the variable lag time between application and detection in the rivers makes it difficult to plan when to take samples. Also, there is little information about the persistence of pesticides in the environment. Therefore, an intensive and continuous sampling effort is needed to adequately define pesticide sources over time and to determine the transport of the pesticides within the delta-bay system. By using this information about the behavior of pesticides in the environment, planners and resource managers, as well as the farmers in the Central Valley, will be able to make informed decisions about the use of pesticides relative to the effects on the environment.

Ground Water Atlas of the United States

By James A. Miller

Grou

round water is an important national resource. Planners, managers, and personnel from Federal and State agencies, congressional staffs, and the public routinely request information from the USGS about aquifers and ground-water resources. Whereas the USGS and other Federal and State agencies have collected extensive amounts of ground-water data for more than 80 years, there has not been a comprehensive overview of the Nation's ground-water resources on a national scale until now. When complete, the new Ground Water Atlas of the United States will provide in a single publication the most important ground-water information collected by the USGS. The Atlas will also serve as a basic reference that describes the location and the geographic, geologic, and hydrologic characteristics of all of the major aquifers in the Nation.